Introduction: Kidney transplant recipients (KTRs) are highly vulnerable to severe COVID-19, however are poorly protected by vaccination. Additional vaccine doses have achieved limited improvements in serological neutralisation or T cell response. A novel strategy to boost vaccine response is needed. Inhibition of mechanistic target of rapamycin (mTOR) has been found to enhance the formation of memory T cells following vaccination of non-human primates.

Methods: KTRs and age-matched healthy controls (HC) were recruited from a transplant centre in South Australia to undergo a 2-dose vaccination schedule with BNT162b2 or ChAdOx1. KTRs were receiving standard-of-care (SOC) triple therapy (tacrolimus, mycophenolate mofetil, prednisolone; n=15), sirolimus-inclusive (n=15), or everolimus-inclusive (n=11) protocols. Patients on mTORi-inclusive protocols were most commonly receiving sirolimus or everolimus, mycophenolate mofetil, and prednisolone. Following 2 vaccine doses, spike-specific IgG and T cell responses (by IFNγ ELISpot) were measured to assess vaccine immunogenicity, and live virus neutralisation and anti-receptor binding domain (RBD) IgG (Elecsys, Roche) were evaluated as correlates of protection from infection and disease. Function and phenotype of antigen-specific T cells were further interrogated by flow cytometry.

Results: KTRs on mTORi-inclusive protocols demonstrated improved humoral immunity, with 46.7% and 63.6% seroconversion rates in the sirolimus and everolimus groups, respectively, compared with 26.3% in the SOC group. This was mirrored by improved serological neutralisation of live SARS-CoV-2 virus (ancestral strain) by patients on mTOR inhibitors. Serological neutralisation of the Omicron variant was achieved by 20% of KTRs on sirolimus, but by none in the SOC nor everolimus groups. Remarkably, sirolimus use was associated with a median antiviral T cell response 55-fold greater than SOC therapy, and 5-fold greater than HCs. SARS-CoV-2-specific CD4+ and CD8+ T cells in these patients were highly polyfunctional and formed robust central memory out to 3 months post second vaccine dose. While the phenotype of antigen-specific memory T cells from KTRs on SOC therapy was skewed relative to healthy individuals, this abnormality was corrected in the sirolimus group.

Conclusions: These data support a randomised controlled trial of immunosuppression modification with sirolimus as a strategy to directly improve vaccine responses in KTRs.